A test and evaluation circuit of the above-mentioned kind is described in European Patent No. 0 060 992. Proximity sensing occurs when an external actuating element comes in proximity to the oscillator causing its oscillations to be damped. In the disclosed circuit, a test and evaluation circuit for determining the proper functioning of the proximity switch, which is provided as an element of a machine control, is described. The proximity switch can be switched between its two switching states, both when its oscillator is damped and undamped, by means of a control circuit. Thereby, a clocked output signal is generated at the output of the proximity switch, which signal is fed to an evaluation circuit connected to the proximity switch. As long as there is a clocked output signal, an enable signal is given by the proximity switch via its relay circuit monitoring the machine control. If the clocked output signal fails to arrive however, a blocking signal for the machine control is generated. In the control circuit of the proximity switch described therein, a square wave signal is generated continuously by means of an astable multivibrator. If the proximity switch is damped for an extended period of time by an external actuating element, thermal overload protection is required in order to protect electronic switching elements in the evaluation circuit. In addition, in the externally damped state of the proximity switch, its evaluation circuit is stressed by current. This current stress is of particular concern to the switching transistors as well as the primary side of a transformer located at the input of the relay circuit. The relay circuit contains two relays which are switched one after the other and are located in different current paths.
West German patent No. DE 33 42 710 discloses an inductive proximity switch with an RF oscillator stage, a switching stage, an amplifier stage and an output stage. Electric energy is fed to the proximity switch at its two terminal points through an evaluation unit. By means of a bridge rectifier which is inserted between the terminal points and the circuit components, the proximity switch can be operated independently of the polarity of the circuit supplying signal current. If the oscillator is damped externally, it is possible to provide a damping reduction of the oscillator by reversing the polarity of the signal current circuit, which, if the circuit components operate properly, results in a corresponding signal change at the output of the proximity switch. Similarly a functional test can be carried out by means of reversing the polarity of the signal current circuit if the oscillator is not damped externally. This leads to internal damping of the oscillator. For switching between interrogation operation which represents the normal current state, and test operation, certain requirements as to a supply, evaluation and monitoring circuit must be met. Such a circuit must make available the supply voltage. It must also carry out the periodic polarity reversal of the supply voltage in accordance with a pulse sequence. This pulse sequence which is used for the switching reversal, however, is generated by the evaluation and monitoring circuit.
Finally, a circuit arrangement for an inductive proximity switch is known from West German Patent No. DE 31 50 212, by means of which the functioning of the proximity switch can be checked regardless of the actual operating condition of the oscillator. The proximity switch comprises a high-frequency oscillator, a subsequent amplifier and a testing device with a test terminal. By addressing the test terminal with a test signal triggered via a push button, the oscillator, if not damped from the outside, can be damped, and, if damped, the oscillator can be relieved of damping. This reversal of the instantaneous oscillator state caused internally to the circuit by means of the push button causes a change of the output signal present at the proximity switch. Its evaluation provides information regarding the availability of the proximity switch. The function of the push button can be taken over by a circuit stage which responds automatically.
There is a need, however, to provide a test and evaluation circuit for a proximity switch which is improved over the aforementioned prior art, particularly of the type disclosed in European Patent No. 0 060 992, by means of which trouble-free operation of the proximity switch itself, and of the elements of the test and evaluation circuit, is monitored, whereby the reliability of the test and evaluation circuit and thereby, also the availability of the proximity switch is increased.